<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .search {
            position: relative;
            width: 178px;
            margin: 100px;
        }

        .con {
            display: none;
            position: absolute;
            top: -40px;
            width: 171px;
            border: 1px solid rgba(0, 0, 0, .2);
            box-sizing: 0 2px 4px rgba(0, 0, 0, .2);
            padding: 5px 0;
            font-size: 18px;
            line-height: 20px;
            color: #333;
        }

        .con::before {
            content: '';
            width: 0;
            height: 0;
            position: absolute;
            top: 28px;
            left: 18px;
            border: 8px solid #000;
            border-style: solid dashed dashed;
            border-color: #fff transparent transparent;
        }
    </style>
</head>
        <div class="search">
            <div class="con">123</div>
            <input type="text" placeholder="请输入单号" class="jd">
        </div>
<body>
    <script>
        /* 
        快递单号输入内容时，上面得大号字体盒子(con)显示(这里面得字体号更大)
        表单检测用户输入：给表单添加键盘事件
        同时把快递单号里边的值(value)获取过来赋值给con盒子(innerText)作为内容
        如果快递单号里边的内容为空，则隐藏大号字体的盒子(con)盒子
        */
    var con = document.querySelector('.con');
    var jd_input = document.querySelector('.jd');
    jd_input.addEventListener('keyup',function() {
        if(this.value == '') {
            con.style.display = 'none';
        } else{
            con.style.display = 'block';
            con.innerHTML = this.value;
        }
    })
    </script>
</body>

</html>